The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. The feed (e.g. natural gas, associated gas and/or coal-bed methane, residual (crude) oil fractions or coal) is converted in a gasifier, optionally in combination with a reforming unit, into a mixture of hydrogen and carbon monoxide. This mixture is often referred to as synthesis gas or syngas.
The synthesis gas is then fed into a Fischer-Tropsch reactor where it is converted in a single step over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight compounds comprising up to 200 carbon atoms, or, under particular circumstances, even more.
The hydrocarbons formed in the Fischer-Tropsch reactor typically proceed to a hydrogenation unit, optionally a hydroisomerisation/hydrocracking unit, and thereafter to a distillation unit.
The ratio of hydrogen to carbon monoxide produced by a gasifier is typically less than the optimum ratio preferred in a Fischer-Tropsch reactor. The hydrogen concentration in the gasifier synthesis gas can be increased by, for example, a Steam Methane Reformer (SMR) which can convert methane and steam to synthesis gas with a hydrogen:carbon monoxide ratio of around 5:1 to 7:1. This SMR synthesis gas can be used to increase the relative hydrogen content of the synthesis gas from a gasifier before it proceeds to a Fischer-Tropsch reactor. Typically, a SMR also produces sufficient hydrogen for use in other units or processes which are integral or associated with the Fischer-Tropsch process, for example a hydrogenation unit.
The steam reforming reaction is endothermic and the heat required for this reaction is typically provided by combusting a fuel gas. A combustion exhaust gas or flue gas is thus produced. Flue gas produced by a steam methane reformer or by gas turbines typically comprises water vapour, carbon dioxide, carbon monoxide, nitrogen, optionally small amounts of C2-C6 hydrocarbons and other gases. Individual components, in particular carbon dioxide, may be separated from the flue gas for subsequent sequestration.
In the prior art, many processes for separating carbon dioxide from flue gas or combustion exhaust gas are described. In EP 551 876, for example, is described a process for removal and recovery of carbon dioxide from combustion exhaust gas leaving a boiler. The process of EP 551 876 aims to reduce the decrease in overall power generation efficiency due to the recovery of carbon dioxide.
Before separation of carbon dioxide or other components, the flue gas is typically cooled using water. The cooling process results in water evaporation and so pure make-up water must be continuously added. The provision of pure water adds costs to the process, particularly in a hot climate where water availability is limiting.